A conventional technique has been known in which a data is made redundant and can be quickly recovered in case of failure by storing an original data and a backup data of the original data in a distributed manner in a plurality of data disks of a storage device (see, for example, Patent Document 1). In such a storage device, a data disk has plural logical storage sections which include a primary area in which an original data is stored and a backup area in which a backup data of the original data is stored. The storage device is equipped with a plurality of data disks each having a primary data area and a backup data area. The storage device stores an original data in a primary data area of one data disk (also referred to as a disk on a primary side) and also stores a backup data of the original data in a backup data area of another data disk (also referred to as a disk on a backup side). This makes it possible for a storage device of this type to quickly recover data in case of failure of a disk on the primary side or a disk on the backup side, given both the disk on the primary side and the disk on the backup side are constantly rotated.
Specialized storage devices having a large number of data disks for storing therein enormous amounts of data have been used for various purposes these years in order to support a current information society such as, for example, the Internet. The size and the number of such storage devices have been increased year by year in response to increasing amounts of information. In the storage device, a write request for writing a data to be stored or a read request for reading a stored data has also been increasing. On the other hand, power consumption of the storage device has also been increased. Reduction in the power consumption and management cost of the storage device has now been regarded as a pressing problem with respect to a measure against global warming.
In order to make the storage device consume less power, the most effective way is to stop rotation of a magnetic disk device (a data disk) in which data is stored as long as possible. It is concerned, however, that the stop of rotation of a magnetic disk device could degrade its performance in regard to a response time and a throughput. Therefore, various techniques according to the related art have been proposed so as to address the problem of maintaining performance of a magnetic disk device while saving its power consumption.
Some of the major techniques according to the related art are known as follows. In one of the techniques, if there are plural data disks, making use of the nature that accesses are not made uniformly to all of the data disks, a data which is frequently accessed is collected into a limited number of data disks. In another technique, if a data disk is in an idle state in which the data disk is rotating but is not performing a write processing or a read processing, the rotation speed of the data disk is kept low. In other technique, in addition to a main data disk in which an original data is stored, a cache disk for access in which a copy of the original data is stored is prepared. An appropriate cache algorithm is devised, to thereby reduce frequency of accesses to the data disk.
For example, Non-Patent Document 1 proposes a technique in which frequently-accessed data are aggregated in a small number of read-only cache disks, making use of locality of data accesses, and most other data disks are brought into a stop state as long as possible without making the data disks rotate.